Method and apparatus for measuring the electrical impedance properties of geological formations using multiple simultaneous current sources

ABSTRACT

A system for measuring geological data is disclosed. The system includes several transceivers distributed over a geographical area. Each of the transceivers has at least one transmitter and at least one receiver. The transceivers are in communication with each other. The receivers are adapted to measure at least one electrical signal. The transmitters are adapted to inject an electrical current into a subsurface area. The transmitters operate simultaneously to inject the electrical current into the subsurface area simultaneously from a number of locations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 61/448,512, filed Mar. 2, 2011, and U.S. patent application Ser. No.13/409,482, filed Mar. 1, 2012, the entire contents of which are hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates to measuring subsurface electrical properties inorder to obtain geological information and more particularly to using adistributed system of low-power transceivers to perform large scalegeological surveys.

BACKGROUND

Several techniques and methods have been used to measure subsurfaceelectrical properties in order to obtain geological information aboutunderground structures. There are three closely related electricalgeophysical techniques (EGT); electrical resistivity tomography (ERT),electrical impedance tomography (EIT) and controlled sourceelectromagnetics (CSEM). These techniques seek to determine subsurfaceelectrical properties and operate at audio or sub audio frequencies.

Measurements are made by inducing current flow through a pair ofelectrodes and simultaneously monitoring induced voltages in additionalpairs of electrodes and their connecting wires. The electrodes areformed from metal or graphite placed either directly in the ground orplaced inside a porous container containing a salt solution which inturn is placed in the ground. Because of the simple, robust nature ofthe electrodes, EGT systems are amenable to either characterization modewhere single surveys are made to locate subsurface features, or as amonitoring tool where the electrodes are permanently placed and thetechnique is used to monitor the changes in the subsurface over time.

The primary differences between the techniques are in the interpretationmethods applied to the resulting data. The ERT technique assumes thatthe electrical potential can be accurately approximated using asteady-state approximation based on Laplace's equation. The EITtechnique assumes that the intrinsic properties are frequency dependentbut that the electrical potential can still be approximated using acomplex Laplace's equation. The CSEM method requires a full solution toMaxwell's electromagnetic equations.

These methods provide tools for imaging subsurface electricalresistivity distributions. Because of this imaging capability, they canalso be used for inferring fluid flow and transport. They can be appliedto a range of depths, well spacings, and reservoir types, and could beused to monitor oil field stimulation applications, such as water andsteam flooding. Anything that changes the electrical resistivity of thesubsurface area can be monitored. Because electric current flows throughthe pore fluid of an underground reservoir, electrical methods areespecially sensitive to pore fluid content and have been applied tomineral exploration, environmental, oil field and industrial projects.

U.S. Pat. No. 7,805,249 purports to disclose a method for performing acontrolled source electromagnetic survey of a subterranean region. U.S.Pat. No. 7,773,457 purports to disclose a wireless exploration seismicsystem that acquires seismic data using a data acquisition module thatcollects seismic data and forwards the data to a central recording andcontrol system. U.S. Pat. No. 7,386,402 purports to disclose anapparatus for transmitting and detecting geophysical data usingreconfigurable control units.

SUMMARY OF ONE EMBODIMENT OF THE INVENTION Brief Description of oneEmbodiment of the Present Invention

In one embodiment, the present invention comprises a system formeasuring geological data. The system includes a plurality oftransceivers with each of the transceivers having at least onetransmitter and at least one receiver. The transceivers are incommunication with each other. The receivers are adapted to measure atleast one electrical signal. The transmitters are adapted tosimultaneously inject an electrical current into a subsurface area.

In another embodiment, the present invention comprises a method ofmeasuring electrical properties of a geological formation. The methodincludes deploying an array of transceivers over a geographical area.Each of the transceivers has at least one transmitter and at least onereceiver. Current is transmitted simultaneously into the geologicalformation from the array of transceivers. Several electrical signals aresensed and the electrical signals are processed into a set of processeddata. The set of processed data is transmitted.

In an additional embodiment, the present invention comprises a systemfor measuring geological data. The system includes means forsimultaneously transmitting an electrical current into a geologicalformation through a plurality of electrodes and means for sensing aplurality of electrical signals resulting from the electrical current.The apparatus also has means for recording the electrical signals andmeans for processing the electrical signals into a set of data. Meansfor transmitting the set of data are also included.

The above description sets forth, rather broadly, a summary of oneembodiment of the present invention so that the detailed descriptionthat follows may be better understood and contributions of the presentinvention to the art may be better appreciated. Some of the embodimentsof the present invention may not include all of the features orcharacteristics listed in the above summary. There are, of course,additional features of the invention that will be described below andwill form the subject matter of claims. In this respect, beforeexplaining at least one embodiment of the invention in detail, it is tobe understood that the invention is not limited in its application tothe details of the construction and to the arrangement of the componentsset forth in the following description or as illustrated in thedrawings. The invention is capable of other embodiments and of beingpracticed and carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein are for the purposeof description and should not be regarded as limiting.

Advantages Of One Or More Embodiments Of The Present Invention

The various embodiments of the present invention may, but do notnecessarily, achieve one or more of the following advantages:

The ability to provide an apparatus for measuring geological data withimproved resolution and superior signal to noise ratios;

The ability to provide a modular system of transceiver devices that cancollect data as a standalone unit or can be configured into an array;

The ability to provide an array of transceiver devices that can injectelectrical current from multiple transmitters simultaneously throughmultiple electrodes into the ground;

A system of distributed transceivers with the ability to transmitcurrent simultaneously from a number of transceiver locations;

The ability to provide flexibility in the transmission and sensing ofarray patterns;

The ability to collect and process data in multiple waveform modes;

The ability to provide a geological measurement system containingmultiple transmitters each capable of transmitting precisely controlledwaveforms and multiple transceivers that can transmit simultaneously;

The ability to provide a transceiver apparatus that can communicate witheach other through either a wired, fiber optic, or wireless radiofrequency or infrared interface; and.

These and other advantages may be realized by reference to the remainingportions of the specification, claims, and abstract.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is substantially a perspective view of a transceiver apparatus orunit in accordance with the present invention.

FIG. 2 is substantially a schematic diagram of the components of thetransceiver of FIG. 1.

FIG. 3 is substantially a more detailed schematic diagram of a portionof the transceiver of FIG. 2.

FIG. 4 is substantially a diagram of an array of linked transceivers.

FIG. 5 is substantially a flowchart of a method of measuring theelectrical impedance properties of a geological formation using an arrayof transceivers in accordance with the present invention.

FIG. 6 is substantially a graph of current versus time illustrating thewaveform used for time-domain induced polarization or electromagneticmeasurements in accordance with the present invention.

FIG. 7 is substantially a contour map of sensitivity for a prior artelectrical geophysical technique (EGT) system.

FIG. 8 is substantially a contour map of data sensitivity to voxels at adepth of 200 meters in accordance with one embodiment of the presentinvention.

FIG. 9 is substantially a contour map of data sensitivity voxels at adepth of 200 meters in accordance with another embodiment of the presentinvention.

DESCRIPTION OF CERTAIN EMBODIMENTS OF THE PRESENT INVENTION

In the following detailed description of the embodiments, reference ismade to the accompanying drawings, which form a part of thisapplication. The drawings show, by way of illustration, specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

FIG. 1 illustrates an individual transceiver apparatus, device or unit20. Transceiver 20 comprises a GPS antenna 1001, keyed switch 68,communication antenna 70, status display 60, link input terminals orconnectors 1005A and 1005B, output terminals or connectors 1006A, 1006Band 1006C, removable battery unit 1011 and transmitter heat sink 1012.In one embodiment, the transceiver 20 is connected via a male connectorportion 56A and cable 32A to an electrode 24A through a connector 30A.Transceiver 20 is also connected to a second electrode 24B viaconnectors 30B and cable 32B and male connector portion 56B. Theelectrodes 24A, 24B are composed of electrically conductive materialsuch as metal, graphite, carbon composite or metal filled composite. Theelectrode can be rod shaped to be inserted into the ground or flatplates placed on the surface of the ground. Electrodes 24A and 24B canform a dipole 25. In this embodiment, the cables 32A, 32B are strandedcopper wire with a plastic insulated sheathing and soldered, braised,clamped, or glued to the electrodes 24A, 24B using a conductive polymer.Transceiver apparatus 20 can be operated completely independently or canbe linked together to form an array or grid of transceiver apparatuses.If operated independently, transceiver apparatus 20 may be programmed totransmit electrical signals and/or measure electrical signals at apredetermined time or during predetermined time intervals.

Power and control unit 50 can include a housing 52 that contains theelectronic components that form transceiver apparatus 20. Housing 52 hasseveral sides 54 that form the outer surface of housing 52. Maleconnector portions 56A and 56B can be received by output connectors1006A and 1006B and can mate with cables 32A and 32B. Input connectors1005A-B and output connectors 1006A-C can be mounted to one side 54.

An antenna 70 can be used to create a wireless link between severaltransceiver apparatuses 20 or a remote computer 150 (FIG. 2) used forcontrol and downloading data. Alternatively, connector 1010 can be usedto connect an external terminal or computer to the unit to send a seriesof pre-programmed command or to retrieve data. A display 60 such as aliquid crystal display (LCD) can be mounted to one side 54. Display 60can display operating parameters such as battery voltage and temperatureA power switch 64 is mounted to housing 52 and can be used to turn poweron and off to transceiver apparatus 20. In one embodiment, the switch isrotary type with a removable key that can be removed to prevent the unitfrom being turned on or off accidentally. An indicator light 68 may beincluded with transceiver apparatus 20 in order to indicate variousoperating parameters or convey other information to a user. For example,if transceiver apparatus 20 experiences a fault, indicator light 68 maybe flashed.

With reference to FIG. 2, further details of transceiver apparatus 20are shown. Transceiver apparatus 20 comprises a communications/storagemodule 80, a central processor unit 110, a timing module 86, atransmitter 100, a power source or unit 84, a receiver module 90 thatmonitors electrical potential, a multiplexer module 120, electrodes 24and computer server 150.

Communication/storage module 80 can be a low power wireless module suchas the Digimesh™ system that allows peer-to-peer communication betweenmultiple wireless modules or with a computer server. The Digimesh™system is commercially available from Digi Corporation of Minnetonka,Minn. Communication/storage module 80 can provide wirelesscommunications and control the system power. Communication/storagemodule 80 is connected with antenna 70. Communication/storage module 80can wirelessly communicate with computer server 150 or with othertransceiver apparatuses 20 using a wireless signal 72 in order toexchange data, instructions and operating parameters.

Communication/storage module 80 can send and receive wirelesscommunication signals 72. Communication/storage module 80 can use a widevariety of communication protocols and systems and can include, but isnot limited to PCS, GSM, TDMA, CDMA, Internet Protocol (IP) network,Wireless Application Protocol (WAP) network, a WiFi network, bluetoothor a local area network (LAN). In another embodiment,communication/storage module 80 may be a wireless module, a direct wirelink (for example Ethernet or RS485), a fiber optic link, or an infraredlink. In one embodiment, communications/storage module 80 can be a datastorage device. In this embodiment, each data storage device wouldcollect data according to a set of commands stored on the data storagedevice. After the data collection is completed, data would be retrievedfrom individual transceivers.

Communication/storage module 80 allows each transceiver apparatus 20 tocommunicate at distances of a few hundred meters even in an urbanenvironment. Communications can travel or hop from one transceiverapparatus 20 to another transceiver apparatus 20 and to others allowingcommunications over several kilometers in urban environments and tens ofkilometers in rural environments. The communication rate can slow downif the data is required to travel through a large number of transceiverapparatuses 20.

The central processor unit (CPU) 110 can be a low-power microprocessorthat is commercially available. CPU 110 can be in communication withcommunication/storage module 80, receiver 90, transmitter 86 andmultiplexer 120 through a data bus 114. CPU 110 can contain a machinereadable medium or memory unit 112. Memory unit 112 may be internal toCPU 110 or may be external memory such as flash memory, ROM, RAM or ahard drive unit. CPU 110 is connected to memory unit 112. Softwareinstructions and programs 113 may be stored in memory unit 112 forexecution on CPU 110 in order to control the operation of transceiverapparatus 20.

The machine-readable medium 112 on which is stored one or more sets ofinstructions such as software programs 113 can that include any one ormore of the methodologies or functions described herein. The software113 may also reside, completely or at least partially, within the CPU110. Software 113 may also reside or be transmitted from computer server150. The software 113 may include data objects and applications that canbe transmitted or received over to or from computer server 150 viawireless signal 72.

Computer server 150 can also include a machine readable medium or memoryunit 152 that contains software programs or instructions 154. Memoryunit 152 is connected to computer server 150. Software 154 can betransmitted from computer server 150 to CPU 110 and memory unit 112.

While the machine-readable medium 112 is shown in an example embodimentto be a single medium, the term, “machine-readable medium”, should betaken to include a single medium or multiple medium such as acentralized or distributed database, and/or associated caches andservers that store one or more sets of instructions. The term,“machine-readable medium”, shall also be taken to include any mediumthat is capable of storing, encoding or carrying a set of instructionsfor execution by the machine and that cause the machine to perform anyone or more of the methodologies shown in the various embodiments of thepresent invention. The term, “machine-readable medium”, shallaccordingly be taken to include, but not be limited to, solid-statememories, optical and magnetic media, and carrier wave signals.

To compensate for potentially slow communications, the CPU 110 canperform most of the data processing, and can perform a series ofpre-scheduled tasks so that computer server 150 needs only to downloadschedule and timing information to each transceiver apparatus 20 ratherthan detailed control information. Computer server 150 can also uploadprocessed data from transceiver apparatuses 20 as opposed to uploadingraw unprocessed data. By programming CPU 110 to process data at eachtransceiver apparatus 20, the need for rapid communications and highdata throughput rates is reduced.

Power source or module 84 can be any suitable source of power. Forexample, power source 84 can be a high discharge battery and a powercontroller that distributes power to the various components transceiverapparatus 20 through a power bus 85. In other embodiments, power source84 can be solar panels or a connection to an external power source or acombination of these power sources.

The system timing unit 86 provides a common time reference to alltransceiver apparatuses. The system timing unit 86 can be an embeddedglobal positioning system (GPS) module. System timing unit 86 is incommunication with transmitter 100 through data bus 114. In anadditional embodiment, system timing unit 86 can be a high precisioncrystal clock or a timing signal sent from a central unit either througha direct wire link or a radio frequency link.

Transmitter 100 can inject a current through dipole 25 including one ormore of electrodes 24A, 24B into the ground. Transmitter 100 is incommunication with CPU 110 through data bus 114 and is also connected tomultiplexer 120. Receiver 90 is in communication with CPU 110 throughdata bus 114 and is also connected to multiplexer 120.

While transceiver apparatus 20 was shown with one receiver 90 and onetransmitter 100, more or fewer receivers and transmitters can beincluded within transceiver apparatus 20. In an embodiment, transceiverapparatus 20 may include only receivers or only transmitters. In anotherembodiment, some transceiver apparatuses 20 may have multipletransmitters. In one embodiment, some transceiver apparatuses 20 mayhave multiple receivers.

Multiplexer 120 can be a 3×4 multiplexer that allows several receivers90 and transmitters 100 to be interconnected with electrodes 24 and forcommunication signals to be routed from one transceiver to the nexttransceiver. Electrodes 24A and 24B are connected to multiplexer 120.The electronic components that form transceiver apparatus 20 can beconnected, packaged and manufactured using conventional electronicassembly techniques such as printed circuit boards, cables, connectorsand surface mount assembly techniques.

A number of transceiver apparatuses 20 can be deployed over ageographical area in an array configuration. The array may have aregular order such as in a grid or may have a more random orientation.Computer server 150 can control an entire system of deployed transceiverapparatuses 20. Computer server 150 can have a wireless or internetconnection that allows real-time monitoring of the system configurationand uploading and imaging of the data from other remote locations.

Software 113 and 154 can be programmed to verify the configuration ofthe system of deployed transceiver apparatuses 20. This can be doneusing timing or GPS unit 86 in the system. The connections betweentransceivers apparatus 20 can also be tested and confirmedautomatically. To do this, the impedance is monitored in one transceiverwhile the interconnections are enabled and disabled in adjoiningtransceivers using multiplexer 120.

Turning now to FIG. 3, additional details of transceiver 20 includingreceiver 90, transmitter 100 and multiplexer 120 are shown. In thisembodiment, the receiver 90 can further comprise two receiver channels90A, and 90B that allow for the collection of two measurements at thesame time or simultaneously. Each receiver 90A and 90B has a separateinput terminal Input and a ground terminal Gnd shared with the otherreceiver and transmitter. Transmitter 100 has a terminal Iref used tomonitor current flow across a precision resistor that is internal totransmitter 100, an output terminal Output and a ground terminal Gnd.The connector 1005 of one transceiver can be connected to the outputconnector 1006 of another transceiver 20 to allow communication signalsor electrical connections to be routed from one transceiver 20 to thenext. The connector 1005 is connected to the output connector 1006 ofother transceivers 20 using a cable 210.

In the embodiment of FIG. 3, the multiplexer 120 contains 15 controlledswitches, 1016 through 1030. Switches 1016-1030 can be latching ornon-latching mechanical or optical relays or relay modules and arecontrolled by the system microprocessor. Two of the switches, 1016 and1017 work in tandem to switch the transceiver 20 from transmit mode toreceiver mode. In FIG. 3, both switch 1016 and 1017 are shown in thereceiver position. In this position the transmitter output isdisconnected and the receiver 90B is connected to the 3×4 multiplexer120. By closing the appropriate switches, the input and ground leads ofthe receiver 90B can be connected to any of the three connectors 1006A,1006B and 1006C. Connectors 1006A-1006C may be connected to one or moreelectrodes 24 or to the input of another transceiver 20.

When the transceiver 20 is operating in transmit mode, both switches1016 and 1017 are switched to the alternate position. The transmit mode,allows the transmitter 100 to be connected to any of the outputconnectors 1006A-C. Switch 1018 enables or disables the second receiverchannel 90A. Alternately, switch 1018 enables or disables a connectionto input connector 1005.

In the position shown in FIG. 3, the receiver 90A channel is disabledand the connection to input connector 1005 is enabled. Enabling inputconnector 1005 allows another transceiver apparatus 20 to be connectedto any of the 3 outputs connectors 1006A-C. Connector 1005A is connectedto the 3×4 multiplexer 120 that allows electrical potential orelectrical current from another transceiver 20 to be coupled or routedto any of the terminals of the output connector 1006A-C.

Each receiver 90A, and 90B can include a twenty-four bit analog todigital converter. When transmitter 100 is transmitting, one of thereceivers' 90B can monitor the current flow. Transmitter 100 can controlits own current flow with about twenty bits of precision. Therefore,monitoring current using the receiver twenty-four bit analog-to-digitalconverter provides better accuracy and a better method of calibratingthe transmitter output.

Multiplexer 120 can be connected with receivers' 90A and 90B,transmitter 100 and one or more electrodes 24.

Multiplexer 120 can provide a flexible interconnection between multipletransceiver apparatuses 20. Multiplexer 120 can connect or link togetherseveral different transceiver apparatuses 20.

FIG. 4 illustrates several transceiver apparatuses 20 linked together oroperating cooperatively to form a transceiver system or array 200.Transceiver array 200 can be deployed in a grid formation over ageological area. Alternatively, transceiver array 200 can be arranged ina line or in a linear manner. Transceiver array 200 comprisestransceiver apparatuses 20A, 20B, 20C, 20D and 20E. Each individualtransceiver apparatus 20 can communicate with another transceiverapparatus 20 either through the use of multiplexer 120 and electricalcables 210 or wirelessly using communication/storage module 80. In FIG.4, transceiver apparatus 20E is shown operating independently.

With linked transceivers 20A to 20D, the ground electrode 24 for onetransceiver can be used as the remote electrode 24 for the linkedtransceiver, or the signal can be passed on to the next remote electrode24 or linked transceiver. The present invention provides an enormousamount of flexibility in the transmission and sensing of array patterns.FIG. 4 shows an example of linked transceivers. Each transceiver canlink to one (20A, 20C) or more (20B, 20D) transceivers or can functionas a stand-alone transceiver (20E).

Transceiver array 200 can be arranged over a geographical area and suchthat current is transmitted simultaneously from a number of transceiverlocations. Transceiver array 200 can be used to collect audio or subaudio frequency electrical geophysical data. The data can be collectedeither under direct user control or autonomously by following a set ofpre-assigned configuration commands. The geophysical data can beinterpreted and analyzed using a multi-dimensional data inversion andinterpretation program. Geophysical data from transceiver array 200 canbe transmitted to computer server 150 (FIG. 2). Computer server 150 canperform calculations and analysis to provide two or three-dimensionalimages of subsurface electrical impedance properties.

In an embodiment, the data from several receivers may also be combinedto make a single data point by taking a weighted average of the datafrom each individual receiver.

Referring to FIG. 5, a flowchart of a method of measuring the electricalimpedance properties of a geological formation using an array oftransceivers is shown. Method 500 includes deploying transceiver array200 (FIG. 4) over a geographical area for which the subsurface featuresare desired to be measured in step 510. At step 520, the systemconfiguration of the transceivers is verified and tested. Step 520 mayinclude transmitting instructions and parameters from computer server150 (FIG. 2) and computer server 150 receiving data about the settingsof each transceiver. Current is injected simultaneously from thetransmitters in transceiver array 200 into the geological formation atstep 530. For example, transceivers 20A-20D of FIG. 4 can simultaneouslyinject current into a geological formation. At step 540, one or morereceivers in each transceiver can monitor or sense and record theresulting electrical signals or voltage levels received by thereceivers. In FIG. 4, transceiver 20E can be the receiver. The data ordataset generated from the sensed electrical signals can be initiallyprocessed by CPU 110 (FIG. 2) within each transceiver at step 550. Theprocessing can include performing initial calculations on the datareceived. The processed data is transmitted to the computer server instep 560. Transmitting of low-frequency electric current requires twoelectrodes, one current source and one current sink; this pair ofelectrodes is referred to as a dipole. For this invention bothelectrodes can be connected to a single transceiver or one electrode canbe connected to one transceiver and the second electrode to a remotetransceiver that is connected to the first transceiver. In oneembodiment of the current invention the current flow and receivedpotentials can be routed through several transceivers to create acontinuous, complete signal.

Transceiver array 200 allows the use of a multi-source transmittermethod, which is the ability to inject electrical current from multipletransmitters simultaneously into a geological formation. This allows anumber of low power transceivers to achieve the signal-to-noise levelscomparable to systems with a single, massive, high power transmitter andto have superior resolution to prior art systems. The present inventionis able to collect data ranging from environmental sites with surveys atscales of a few meters to carbon dioxide sequestration sites with scalesof several kilometers.

Progressing from small-scale to increasingly large scale geologicalsurveys causes the signal levels to decrease and noise levels tend toincrease. Several ways are known to compensate for this problem. Remotereference noise reduction methods can help reduce noise levels. Othermethods of dealing with these problems are increased amounts of signalaveraging, and increasing the current flow from the transmitter. Both ofthese methods of improving signal-to-noise problems are limited bysquare-power relations. For example, using signal averaging to decreasenoise effects by a factor of two typically requires a factor of a fourtimes increase in the number of averages. Therefore, the dataacquisition time also increases by a factor of four.

Doubling the current flow of a transmitter requires quadrupling thetransmitted power because to double the current, the voltage must alsobe doubled. However, most of the transmitted power is dissipated a shortdistance from the transmitting electrodes. Therefore, by transmittingcurrent into the ground simultaneously using several source dipolesusing transceiver array 200 and spacing the electrodes an appropriatedistance, the signal strength will increase proportionally to the numberof transmitters. Transceiver array 200 can inject as much current intothe ground using ten 500 watt transmitters as a single 50 Kilowatttransmitter.

FIG. 6, illustrates a time-domain waveform 240 that can be generated bytransmitter 100 (FIG. 2) and provided to electrode 24 (FIG. 4) in orderto inject a current into a subsurface geological formation. Thetime-domain waveform can be a 50% duty cycle square wave. Thetransmitter outputs a positive current 242 for time t₀. The current 244is turned off for the same length of time between t₀-2 t₀ and then areverse polarity current 246 is transmitted during the time period 2t₀-3 t₀. The current 248 is turned off for a second time period from 3t₀ to 4 t₀. This cycle of current injection can be repeated severaltimes and the results averaged. The time period for t₀ may vary from afew milliseconds to a few tens of seconds. The length of the timeperiods can be the same or may be different. In one embodiment, the timeperiods can all be the same.

The transmitters in transceiver array 200 can transmit simultaneously atthe same time. The shape of the transmitted waveform and timing can bethe same from all of the different transmitters. In an embodiment, thewaveform can be at the same frequency and start and end at the sametime. In another embodiment, the amplitude of the waveform may bedifferent at each transmitter but would be tightly controlled to apre-assigned value.

In one embodiment, the transmitters in transceiver array 200 cantransmit waveforms for current injection with the same shape andfrequency but with different amplitudes. For low frequencyelectromagnetic measurements, the transmitted waveforms may also bephase shifted. The transmitted waves can be one of four types: a timedomain waveform as shown in FIG. 6, a frequency domain waveform witheither a sine wave or a square wave, a finite pulse designed to allowfrequency domain data to be collected at two or more frequenciessimultaneously or a pseudo-random waveform.

Plots of computer simulated subsurface sensitivity can be performed toillustrate the improved ability of the present invention to measurechanges in subsurface electrical properties. In three-dimensionalgeophysical data imaging routines, the subsurface in and adjacent to asurvey area is divided into a series of rectangular parallelepipedregions referred to as voxels. The sensitivity of the i^(th) voxel,G_(i), is the estimate of the amount of change of a specific data valuein volts per ampere for a unit change in one of the parameters and isgiven by the equation:

$G_{i} = {\frac{\partial}{\partial P_{i}}V}$

Where V is the electrical potential in volts per ampere measured betweenelectrodes and P_(i) is the natural logarithm of the electricalconductivity of the i^(th) voxel. For the following examples, each voxelis a 50×50×50 meter cube embedded in a 100 Ohm-meter half-space.

FIGS. 7, 8, and 9 illustrate the sensitivities for a series of voxelscentered at a depth of 200 meters. The calculation of sensitivity valuesis given in references on electrical and electromagnetic modeling ofgeophysical data.

FIG. 7 shows a plot of sensitivity 700 with contours lines showingregions of equal sensitivity 710 for a dipole-dipole array of the priorart with a single transmitting dipole 702 and receiving dipole 704. Thecontour map of FIG. 7 is for the data sensitivity to voxels at a depthof 200 meters for a 200 meter transmitting dipole 702 and receivingdipole 704. The contour interval is 0.05 volts per amp. The sensitivityplots show a dumbbell shaped anomaly with the highest responses belowthe transmitting dipole 702 and receiving dipole 704. FIG. 7 shows thesensitivity pattern for a typical, non-multisource system for comparisonwith multi-source methods. The purpose of this invention is to increasethe amplitude and create improved patterns of subsurface sensitivity.

Referring to FIG. 8, a plot of sensitivity 800 for one embodiment of thepresent invention is shown.

FIG. 8 illustrates an example with five parallel simultaneouslytransmitting dipoles 802, 804, 806 and 810 and a single receiving dipole820. Each of dipoles 802-820 are formed by a pair of evenly-spacedelectrodes 24. In one embodiment, each dipole can be formed by a pair ofelectrodes 24 that are connected to one transceiver 20. In anotherembodiment, a dipole can be formed by having one electrode 24 connectedto one transceiver 20 and another electrode 24 connected to anothertransceiver 20. For example, with additional reference to FIG. 4, adipole can be formed between the electrode 24 connected to connector1006A of tranceiver 20E and the electrode 24 connected to connector1006C of transceiver 20B. The transmitting of low-frequency electricalcurrent uses two of the electrodes in transceiver array 200 aselectrodes.

The contour map of FIG. 8 is the sensitivity of data to voxels at adepth of 200 meters with five parallel 200 meter-spaced transmittingdipoles 802-810 and a single receiving dipole 820. The contour intervalis 0.05 volts per amp.

The shape of the contours or anomalies 830 show that the amplitude ofthe signal received has increased by almost a factor of five. Therefore,the sensitivity at depth shows a large increase when the number oftransmitting dipoles is increased from one to five.

Referring to FIG. 9, a plot of sensitivity 900 for another embodiment ofthe present invention is shown. In FIG. 9, an example is shown with aconfiguration that uses four pairs of dipoles 902, 904, 906 and 908 withtwo pairs having opposing polarities and a single receiving dipole 910.The contour map of FIG. 9 is for data sensitivity to voxels at a depthof 200 meters with four opposed 200 meter-spaced transmitting dipoles902-908 and a single receiving dipole 910. Note that the contour line oranomaly 920 roughly in the center of FIG. 9 has a negative value. Thecontour interval is 0.05 volts per amp.

Dipoles 902 and 904 can have negative electrodes 902A and 904A andpositive electrodes 902B and 904B. Dipoles 906 and 908 can have negativeelectrodes 906A and 908A and positive electrodes 906B and 908B. Thepolarity of dipoles 902 and 904 is opposite the polarity of dipoles 906and 908. The dipole configuration illustrated in FIG. 9 can have astrong response at depth; however, the polarity of the sensitivityvalues 930 are inverted below the transmitting dipoles 902-908. Thischange in the sensitivity patterns makes the combination of these twoarray patterns very sensitive to changes in subsurface resistivity belowthe current sources.

The embodiments of FIGS. 8 and 9 use dipoles that simultaneouslytransmit the same magnitude of current with the same timing/phase anduse a single receiver. It is contemplated that embodiments of thepresent invention can use both arbitrary current flow values and theaveraging of data from multiple receivers. In this case, the response ofthe system, V, is made from the weighted summation of several receiversgiven by the equation:

$V = {\sum\limits_{j}{a_{j}v_{j}}}$

Where v_(j) is the observed electrical potential for a single receiverand a_(j) are weighting coefficients. The choice of the location, numberof receivers and weighting coefficients a_(j), and the choice of thenumber, location, and current strength of the transmitting dipoles willbe designed according to the survey objectives and site conditions.Numerical modeling can be used to estimate the signal level to makecertain that the signal-to-noise levels are sufficiently high in orderto collect accurate data. In addition, sensitivity analysis can be usedto estimate the detectability of specific targets. One measure of thedetectability of the target is the normalized sensitivity Ĝ_(i) given bythe equation:

${\hat{G}}_{i} = \frac{{G_{i}}^{2}}{\sum\limits_{k \neq i}{G_{k}}^{2}}$

where G_(k) are the sensitivities of the voxels throughout the surveyarea. Large values of Ĝ_(i) indicate high sensitivity to a specificvoxel or group of voxels.

The present invention can be used to collect resistivity data from ageological formation in which the frequency is low enough or the timeperiod long enough to ignore electromagnetic coupling effects.Alternatively, the present invention can be used to collectlow-frequency electromagnetic data. In this embodiment, the frequency ofthe primary waveform is high enough such that the maximum separationbetween the receiving and transmitting transceiver apparatuses is of theorder of one skin depth and the frequency is low enough such thatelectrical conductivity dominates over dielectric effects.

The present invention may be used either for subsurface characterizationusing portable transceivers or may be used for long-term monitoringusing permanently or semi-permanently placed transceivers withpermanently placed electrodes.

This present invention allows for the measurement of subsurfaceelectrical properties using a distributed system of low-powertransceivers to perform large scale geological surveys. The distributedtransceivers can transmit current simultaneously from a number oflocations.

The present invention uses a modular system of transceivers that cancollect data as independent stand-alone units or can be interconnectedin a grid or matrix to allow flexible multi-dimensional data collection.Each transceiver can collect and process data in multiple waveformmodes. These modes include time-domain, frequency domain orpseudo-random wave sequences.

The present invention contains multiple transmitters that are eachcapable of transmitting precisely controlled waveforms and multipletransceivers that can transmit simultaneously. The transceiverapparatuses or modules can communicate with each other through either awired, fiber optic, or wireless radio frequency or infrared interface.

Although the description above contains many specifications, theseshould not be construed as limiting the scope of the invention but asmerely providing illustrations of some of the embodiments of thisinvention. Thus, the scope of the invention should be determined by theappended claims and their legal equivalents rather than by the examplesgiven.

We claim:
 1. A method of obtaining electrical data from a geologicalformation, comprising: obtaining a common time reference at each ofplural geological data transceivers; transmitting an electrical currentinto the geological formation from each of the plural geological datatransceivers simultaneously based upon the common time reference at eachtransceiver, the electrical current from each of the plural geologicaldata transceivers being of the same frequency and waveform; and sensingone or more electrical signals as electrical data from the geologicalformation.
 2. The method of claim 1 in which the electrical current fromeach of the plural geological data transceivers is of a differentamplitude.
 3. The method of claim 1 in which the electrical current fromeach of the plural geological data transceivers is of a different phase.4. The method of claim 1 in which obtaining a common time reference ateach of the plural geological data transceivers includes receiving thecommon time reference at each geological data transceiver from a globalpositioning system.
 5. The method of claim 1 in which sensing one ormore electrical signals as electrical data from the geological formationincludes sensing plural electrical signals and the method furtherincludes generating a weighted sum of the plural electrical signals. 6.The method of claim 1 in which sensing the one or more electricalsignals as electrical data from the geological formation is performed byone or more other geological data transceivers that are substantiallythe same as, but operated differently from, the plural geological datatransceivers.
 7. A method of obtaining electrical data from a geologicalformation, comprising: obtaining a common time reference at each ofplural geological data transceivers; transmitting an electrical currentinto the geological formation from each of the plural geological datatransceivers simultaneously based upon the common time reference at eachtransceiver, the electrical current from each of the plural geologicaldata transceivers being of the same frequency and waveform and theelectrical current from one or more pairs of the plural geological datatransceivers being of opposite polarity; and sensing one or moreelectrical signals as electrical data from the geological formation. 8.The method of claim 7 in which the electrical current from each of theplural geological data transceivers is of a different amplitude.
 9. Themethod of claim 7 in which the electrical current from each of theplural geological data transceivers is of a different phase.
 10. Themethod of claim 7 in which obtaining a common time reference at each ofthe plural geological data transceivers includes receiving the commontime reference at each geological data transceiver from a globalpositioning system.
 11. The method of claim 7 in which sensing one ormore electrical signals as electrical data from the geological formationincludes sensing plural electrical signals and the method furtherincludes generating a weighted sum of the plural electrical signals. 12.The method of claim 7 in which sensing the one or more electricalsignals as electrical data from the geological formation is performed byone or more other geological data transceivers that are substantiallythe same as, but operated differently from, the plural geological datatransceivers.